Recently, so called live recording of sound has become very popular amoung amateurs, and since movie cameras and VTR (video tape recorder) cameras have also become popular, it is now desired to perform sound recording corresponding to the pictures of the camera. Namely, it is desired to catch only an objective sound rejecting other sound and improving signal to noise (S/N) ratio and sound clarity. In other word, it has been desired to develop a variable directional microphone system which is capable of receiving sound waves in accordance with the zooming operation of the movie or VTR camera.
Apart from such a microphone for use with a movie or VTR camera, it is convenient if a microphone for a customary tape recorder used for recording conversations in a conference or the like, has a variable directivity because voices of all persons in the conference can be recorded with a sufficient recording level by adjusting the directivity of the microphone depending on the distance between the microphone and the speaker.
To meet the above-mentioned requirement of a variable directional microphone one of the inventors of the present invention (in collaboration with other inventors) has already disclosed a variable directional microphone system in a Japanese Patent Application No. 54-41691 (corresponding U.S. application Ser. No. 142,845, now U.S. Pat. No. 4,308,425, and German application No. P3016018.0). The microphone system disclosed therein comprises three first order sound pressure gradient unidirectional microphones. As will be described with reference to the drawings, in the arrangement of the previously disclosed varible directional microphone system, two microphone units are arranged to face one direction on the same axis, while the remaining one microphone unit is arranged in the opposite direction where the diaphragm of the oppositely directed microphone unit and the diaphragm of one of the frontward directed microphone are in the same plane. These three microphone units are fixedly disposed in a casing arranged to allow sound waves from all directions to pass therethrough. Since the center axis of the oppositely directed microphone unit is spaced from the common center axis of the two frontward directed microphone units, the inner diameter of the casing has to be large enough to be able to contain all three microphone units therein. As a result, the already disclosed microphone system is too bulky to be mounted on a movie or VTR camera. Furthermore, since the already disclosed microphone system is large in size, it is heavy and is difficult to handle.
It has been already disclosed in the above-mentioned patent applications that the output signals of the three microphone units are combined where the combining ratio therebetween is suitably adjusted by means of ganged variable resistors. With this arrangement the proposed microphone system is capable of changing its directivity from nondirectivity or omnidirectivity via first order sound pressure gradient unidirectivity to second order sound pressure gradient unidirectivity. However, when the combining ratio is adjusted so that the microphone systen exhibits second order sound pressure gradient unidirectivity, noise in a low frequency range increases and the system is poor at wind noises because the level of low frequency components of the combined output signal is greatly amplified by an equalizer.
Furthermore, two microphone units, which are used for obtaining second order sound pressure gradient unidirectivity among the above-mentioned three microphone units, must be equal or close to each other in frequency characteristics and directivities in order to obtain desired second order sound pressure gradient unidirectivity in the above-mentioned disclosed parrangement. This means that selection of microphone units with respect to frequency characteristics and directivities has to be performed with high precision, and thus the disclosed arrangement requires a high cost in manufacturing.